Conventionally known are motor-operated injection molding machines which use servomotors for driving a mold clamping mechanism, nozzle touching mechanism, etc.
The motor-operated injection molding machines which utilize a servomotor for the mold clamping mechanism are classified broadly into two categories: a straight-hydraulic type, in which a movable platen fitted with a movable-side mold is linearly moved for mold clamping by driving a ball-nut-and-screw mechanism by means of the servomotor, and a type in which the mold clamping is effected by pushing out the movable platen by means of a link mechanism. In general, the link mechanism of the latter type may be formed of a toggle mechanism or a crank mechanism.
Referring now to the diagrams of FIGS. 4 and 5, the respective operations of mold clamping mechanisms, in which the movable platen is pressed against a stationary platen by means of the link mechanism, will be described. In FIG. 4, which illustrates the operation of a link mechanism using a crank, symbol 1a designates the crank, which is rotated around a support joint Q1 by means of a servomotor M. A driving joint Q2 of the crank 1a is connected to an action joint Q3, on the side of a movable platen mp for use as a movable member through a link 1b. When the crank 1a is rotated in the clockwise direction of FIG. 4 by driving the servomotor M in this arrangement, the movable platen mp linearly moves in the direction of the arrow of FIG. 4 (i.e., toward the stationary platen) to come into contact with the stationary platen, thereby producing a mold clamping force. Thereupon, positioning is effected by commanding the servomotor M to take a position such that the driving joint Q2 is located on a line which connects the support joint Q1 and the action joint Q3, after adjusting the position of the support joint Q1 or the like so that a set mold clamping force is produced in a state (lockup state) such that the support joint Q1, driving joint Q2, and action joint Q3 are situated substantially on a straight line, as indicated by dotted line in FIG. 4. In the state that the set mold clamping force is obtained with the support joint Q1, driving joint Q2, and action joint Q3 arranged on a straight line, as a result, the crank 1a and the link 1b receive a reaction force against the mold clamping force in the lockup state in which they are stretched to their full length and situated on a straight line, so that no rotatory force is applied to the crank 1a. Thus, no external force acts so as to rotate the servomotor.
FIG. 5 is a diagram for illustrating the operation of a mold clamping mechanism using a link mechanism of the (double) toggle type. The principle of operation of this mechanism resembles that of the one using the crank mechanism of FIG. 4.
More specifically, when a ball screw bs is driven by a servomotor M, a toggle head th, which is integral with a nut threadedly engaged with the ball screw bs, linearly moves. As this is done, the movable platen moves toward the stationary platen in a manner such that links 1a and 1b and links 1c and 1d gradually shift their respective postures from bent positions, indicated by full lines in FIG. 5, to stretched positions, indicated by dotted lines. Thereupon, the servomotor is commanded to take a position for a set mold clamping force (position such that a support joint Q1, driving joint Q2, and action joint Q3 are situated on a straight line), after adjusting the position of the support joint Q1 or the like so that the set mold clamping force is produced in a state (lockup state) such that the support joint Q1, driving joint Q2, and action joint Q3 are situated substantially on a straight line, as indicated by each dotted line in FIG. 5. In the state that the set mold clamping force is obtained in this manner, as in the case of FIG. 4, the links 1a and 1b and the links 1c and 1d receive a reaction force against the mold clamping force in the lockup state, in which the links in each set are stretched to their full length and situated on a straight line, so that no rotatory force is applied to the link 1a. Thus, no external force acts to rotate the servomotor. Although the toggle shown in FIG. 5 is of the double-toggle type, a single-toggle type is based on the same principle, so that its description will be omitted here.
Although the prior-art principle of operation in the case where the mold clamping mechanism is driven by means of the link mechanism using the crank or toggle has been described, this link mechanism is also utilized in a nozzle touching mechanism. More specifically, the link mechanism using the crank or toggle is utilized for moving an injection apparatus, as a movable member, toward a stationary mold and pressing a nozzle of the injection apparatus against the stationary platen to produce and maintain a predetermined touching force. This principle of operation of the nozzle touching mechanism is equivalent to that of an arrangement in which the movable platen mp of FIG. 4 is replaced with the injection apparatus (which is moved toward the stationary mold during nozzle touching operation) in the case of the nozzle touching mechanism using the crank mechanism, and to that of an arrangement in which the movable platen mp of FIG. 5 is replaced with the injection apparatus in the case of the nozzle touching mechanism using the toggle mechanism. Since these arrangements are based on the same principle as the ones described with reference to FIGS. 4 and 5, individually, further description thereof will be omitted here.
In performing the mold clamping or nozzle touching operation through the link mechanism using the crank or toggle, as described above, an assigned position for the servomotor is judged such that the one link (or crank) and the other are in the lockup state in which they are stretched substantially to their full length. When the delivery of the position command to the servomotor is finished, the link mechanism is locked up by being positioned in this manner, so that the force from the mold clamping mechanism acting around the axis of the servomotor can be thoroughly removed. Consequently, no load acts on the servomotor, so that only a very small driving current is needed to keep the servomotor in its rotational position. Thus, a predetermined mold clamping state or nozzle touching state can be maintained with a supply of fine current.
According to the motor-operated injection molding machine described above, however, various problems arise in the positioning control for the servomotor due to secular changes such as changes in dimensions of various parts of mechanisms or changes in friction coefficient, which are attributable to fit abrasion, deterioration of relative dimensional accuracy attributable to local temperature changes, etc.
In case of fit abrasion at part of a drive transmission section of the mold clamping mechanism which uses the toggle mechanism or crank for mold clamping, for example, the toggle mechanism or crank may sometimes fail to reach a predetermined position (position for the lockup state), or may possibly move beyond the predetermined position, even when the servomotor is controlled so as to be driven to a preset command position. In such a case, a predetermined mold clamping force or nozzle touching force cannot be obtained or maintained, so that the mold clamping mechanism or nozzle touching mechanism cannot establish and maintain a precise lockup state. Thus, in this state, a reaction force from the mold clamping mechanism or a reaction force based on the nozzle touching force acts around the axis of the servomotor, so that a driving current corresponding to the reaction force must be supplied continuously during the mold clamping period or nozzle touching period, in order to control the servomotor so that its present position is maintained. In some cases, therefore, the servomotor may overheat.